1. Field of the Invention
This invention relates to a control system for an internal combustion engine and particularly to a system for controlling the composition of the gas and fuel mixture used in the combustion process occurring in such an engine. The invention also relates to a method of controlling an internal combustion engine.
A modern engine control system comprises look up tables which are addressed by the outputs of sensors for measuring engine operating conditions such as engine speed and load. In an open loop system, the outputs of the look up tables are used directly for providing control signals for control devices. In a spark ignition engine, the control devices comprise a device for controlling the timing of the ignition sparks and a device for controlling the composition of the fuel mixture. Manufacturing tolerances in the sensors and control devices together with changes which may result from drift and wear of these components in service cause differences between actual values of ignition timing and mixture composition and their designed values. Mixture composition is particularly prone to such differences.
There are many situations where these differences must be kept under control as there exists a finite operating window within which the system must operate to avoid constraints on engine performance. An example of such an operating window is provided by a lean burn spark ignition engine. In a lean burn engine, the combustion mixture contains air and fuel in proportions which are leaner than stoichiometric. This window is illustrated in FIG. 1 which is a graph of emissions of nitrogen oxides (NOx) against air to fuel ratio. As shown, a lean burn engine should operate within a window M between a rich limit AF1 beyond which there is a danger of excessive NOx emissions and a lean limit AF2 beyond which there is a danger of excessive engine roughness. Engine roughness may be defined as variations in work output occurring between successive combustion events or periods. Ideally, a feedback system for such an engine would incorporate both a NOx sensor and a roughness sensor to ensure that both constraints were met and that the engine operates within the window M. However, at present there is no suitable NOx sensor for engine control and such a sensor, if it existed, would add to the cost and complexity of the control system.
It is current practice to operate lean burn engines with a design value for the air to fuel ratio which is spaced by a safety margin from the lean limit (AF2 in FIG. 1) corresponding to the onset of excessive engine roughness. Referring to FIG. 2, there is shown a distribution D1 of actual air to fuel ratios about their design value. As mentioned above, this distribution is caused by manufacturing tolerances and drift and wear of sensors and control devices. This distribution has an approximate spread of M1. There are a number of variables for production engines in service which give rise to different levels of engine roughness at the same air to fuel ratio with the same fuel. These variables include differences between individual engines, differences between ignition systems and atmospheric variables. Referring to FIG. 3, there is shown a distribution D2 for air to fuel ratio for a fleet of production engines for a particular value of engine roughness. This distribution has an approximate spread of M2. In order to ensure that any engine out of a fleet of engines has a very low probability of suffering excessive engine roughness, the design value for the air to fuel ratio must be spaced from the value corresponding to the onset of excessive engine roughness by a safety margin of (M1+M2)/2. As a result of providing this safety margin, the majority of the engines of the fleet are operated with a fuel mixture which is richer than necessary and so the NOx emissions from these engines are higher than necessary. In the future, legislation will require a reduction in the emissions of NOx and this, in turn, will lead to a reduction in the safety margin. This reduction in the safety margin will increase the probability of engine roughness in service and this makes it desirable to provide a roughness feedback.
In U.S. Pat. No. 4,368,707, there is described an engine control system in which the air to fuel ratio is controlled so as to achieve a predetermined and acceptable roughness level. However, there are a number of disadvantages in operating an engine in this way. It is generally undesirable to operate an engine at a roughness level which is close to the limit value corresponding to the onset of excessive engine roughness. If an engine is operated close to this limit value, then a lean excursion, as might occur during rapid throttle opening, could lead to an unacceptably high level of roughness. On the other hand, if the engine is operated with a roughness level which is well spaced from the limit value, there is an increased risk of excessive NOx emission from some engines due to an over rich combustion mixture. Under legislation relating to NOx emissions, it is necessary to demonstrate that engines in service would have NOx emissions within the legal limit. In the case in which the air to fuel ratio is controlled so as to achieve a predetermined roughness value, this is difficult to demonstrate. The nature of the variables which give rise to changes in the air to fuel ratio for a particular roughness level is such that it is difficult to obtain the necessary data. Also, the spread of air to fuel ratios in an engine in which this ratio is controlled to achieve a predetermined roughness value can be greater than the spread in an open loop system. More generally, if the air to fuel ratio is controlled so as to achieve a predetermined roughness level, this removes the freedom of the designer to control the air to fuel ratio in accordance with other constraints.
In the case of a compression ignition engine provided with exhaust gas recirculation, there is a finite operating window for the level of exhaust gas recirculation which is generally analogous to that described above in relation to a spark ignition engine. In the case of a compression ignition engine, this window is bounded by a low level limit corresponding to the onset of excessive NOx emissions and a high level limit corresponding to the onset of production of black smoke particulates in the exhaust gas.